In offset printing, printed images are transferred from printing plates mounted on plate cylinders to a moving web of material by transfer cylinders known as blanket cylinders. Typically, printing blankets are mounted on the blanket cylinders which have a rubber surface for transferring the printed images. Conventional printing plates and printing blankets are rectangular in shape and are mounted in axial gaps extending along the circumferential surfaces of the corresponding plate cylinders and blanket cylinders. One problem with this design is that at high operational speeds the gaps in the plate and blanket cylinders cause vibrations in the printing press which have the effect of varying the optical densities of the printed image. There are a number of other problems associated with this design which affects the quality of the final printed product. Many of these problems have been solved by making the printing blanket tubular in shape having a gapless outer circumferential surface. Furthermore, by substituting the conventional flat printing plate with a tubular printing form having a gapless outer circumferential surface endless printing is possible. Arrangements of this nature however have several drawbacks.
To mount a tubular printing sleeve, i.e., a tubular printing form or a tubular printing blanket, an air canal is provided at one end of a corresponding cylinder on which the sleeve is to be mounted. The canal supplies pressurized air radially outward through a plurality of passages. As the printing sleeve is placed over the passages, the pressure from the exiting air radially expands the printing sleeve enabling it to be axially mounted onto the circumferential surface of the corresponding cylinder. Since the inner circumference of the printing sleeve is slightly smaller than the outer circumference of the corresponding cylinder, once the printing sleeve is mounted it is stressed in tension by the corresponding cylinder to provide a tight pressure relationship between the printing sleeve and the corresponding cylinder. This pressure relationship fixes the printing sleeve on the corresponding cylinder so that there is no relative movement therebetween during operation of the press.
A problem with this arrangement is that air gets trapped at the interface of the printing sleeve and the corresponding cylinder. During operation of the press this trapped air creates a continually advancing wave in front of a nip between the corresponding cylinder and an adjacent cylinder against which it is pressed causing the printing sleeve to bulge. This phenomena is known as printing sleeve procession. It creates defects in the printed product by forming latent double images.
Several attempts have been made to reduce or eliminate sleeve procession, but none have been successful. One attempted solution was to increase interference between the printing sleeve and corresponding cylinder. Another was to change the material combination of the printing sleeve and the corresponding cylinder surface to a combination having higher coefficients of friction. Both of these attempted solutions failed, since the primary cause of procession does not involve slippage of the printing sleeve relative to the corresponding cylinder.
Another attempted solution was to decrease the normal forces between the corresponding cylinder and its adjacent cylinders. Although this solution reduces the rate of procession, it also reduces the quality of print to an unacceptable level. Still another solution was attempted which involved mechanically fixing the printing sleeve to the corresponding cylinder. This attempt was also unsuccessful because the printing sleeve was too thin to withstand the forces required to stop the procession, and hence the printing sleeve would tear.
The deficiencies in each of these attempts are fundamental and cannot be eliminated.